OIL PUMP: TYPES, DRIVE MECHANISM, ELECTRONIC OIL PUMP
OIL PUMP: TYPES, DRIVE MECHANISM, ELECTRONIC OIL PUMP
The oil pump is the heart of the lubrication system. It sucks oil up from the oil pan and forces it around oilways in the engine before the oil drops back into the sump and is recirculated. The oil pump is a highly critical part of the engine – if an oil pump stops working it will lead to expensive engine failure 100% of the time. That’s one reason why the oil pump is so directly driven from the crankshaft.
Oil pumps are situated in the oil pan, or more usually at the front of the engine.
1. Crankshaft Driven
Situated directly at the end of the Crankshaft, this design usually incorporates a cover that sits directly above the Sump.
2. Chain Driven
With more modern vehicles being equipped with Timing Chains, the Oil Pump has followed this trend by being driven by a Chain system.
3. Belt Driven
Equipped with a Pulley rather than a Sprocket, this design is driven by a Rubber Belt, much like a Water or Vacuum Pump.
4. Intermediary Shaft Driven
Driven by an auxiliary source such as a Lanchester Unit or Balance Shaft, this is a compact design most commonly used in VW/Audi/Seat/Skoda
5. Gear Driven Pump
Gear-driven pumps include an intermeshing gear which is predominantly used on Heavy-Duty Diesel applications.
6. Shaft Driven Pump
Incorporating a drive shaft, this type of pump is usually driven off a Camshaft or Intermediary Shaft. These usually have a Spline or Slot fitment on the end of the shaft.
OIL PICKUP PIPE
The oil pump sucks oil up from the sump through a pipe – called a pickup pipe. The pipe’s nozzle sits below the surface of the oil, and it is covered by a gauze filter that prevents large particles from being sucked up into the pump.
If this filter screen was to be completely blocked, then the engine would not pick up any oil and the engine would at great risk of being destroyed. Therefore the pickup pipe may have a bypass valve in case the screen is blocked. In this situation, it’s better than the engine continues to receive oil and we take the chance of damage to the oil pump: an oil pump is considerably cheaper than an engine rebuild.
PRESSURE RELIEF VALVE
Both these pump mechanisms will continue pumping oil up to extremely high pressures. In order to prevent damage to the engine, an oil pump includes a pressure-relief valve that will open when oil pressure becomes too high.
The pressure relief valve is on the output side of the pump. The oil on the output side pushes on a piston, or ball bearing. That piston is held closed by a calibrated spring and once the pressure reaches a determined level, the spring will flex and the piston will unseat, allowing oil to return either to the inlet side of the pump or directly down into the sump.
ELECTRONIC OIL PUMP
An electronic oil pump would be more controllable in relation to engine speed, and engines are now starting to use electronic water pumps. But it’s unlikely that oil pumps will ever be electronic. Pumping cold, viscous oil is hard, and more importantly, the failure of an oil pump is disastrous for an engine. With a mechanical pump there is the guarantee that if the engine is running, the pump is turning. That said, some huge industrial engines and race engines are pre-oiled using an electrical oil pump that runs before engine startup and after shutdown.
TYPES OF OIL PUMP
1. The Gear Type Oil Pump
The most common form of oil pump used to be the gear type. It consisted of two meshing gears, which rotate inside a close-fitted housing. As the gears rotated they carried oil around, against the housing. The meshing of the gear teeth forced the oil out into the pump. This gear-type oil pump produced a positive flow of oil to the various parts of the engine to which it was directed. If the flow was blocked, the oil pressure could increase sufficiently to damage the pump. Alternatively, if the engine oil was particularly thick, the pressure required to force the cold oil through the small bearing clearances could cause a similar pressure build-up, damaging the pump. To eliminate these possibilities an oil-pressure relief valve was fitted. This returned the oil to the sump, or to the oil tank if a dry-sump system was used when the pressure created exceeded a predetermined value.
2. The Rotor Oil Pump
Another form of the oil pump, more common than the gear type, is the rotor variety. This consisted of a rotor, with four or five external lobes, which rotated inside an outer ring (termed a stator) having five or six internal lobes. The axis of the inner rotor was offset from the axis of the outer ring. The effect was that, although the outer ring was driven by the inner rotor, the volume between the lobes varied as the two rotated. Oil was thus caught between the lobes and forced out into the pump discharge tube. The action of this type of oil pump can be likened in some ways to the operation of the Wankel or rotary engine. The rotor-type oil pump also required a pressure relief valve.
3. Eccentric Rotor Oil Pump
The most common type of oil pump in modern cars is the eccentric rotor type, originally made by Hobourn Eaton. The main components are an external and an internal rotor, mounted eccentrically in relation to each other. Each rotor carries a number of lobes, the inner rotor has, say, four lobes mounted on the outside face, while the outer rotor has five lobes facing inwards. The precise configuration differs from the pump to the pump, but the number of lobes on the outer rotor always exceeds that on the inner rotor by one. Both rotors are mounted within the pump body, the most important features of which are the crescent-shaped inlet and outlet ports. A drive from the camshaft rotates the inner rotor which in turn rotates the outer rotor. Oil is drawn in through the inlet port and trapped between the opposing lobes.
As the outer rotor revolves more quickly the clearance between the lobes decreases and pressure is built up until the outlet port is exposed. As the edge of the rotor passes the outlet port, oil is forced under pressure into the lubrication system. Each space formed by the lobes repeats the process and a continuous flow of oil is created. Of all oil pump types, the eccentric rotor pump is the most efficient and is fitted by nearly all major manufacturers. It is normally long-lasting, but wear will eventually occur between the rotors resulting in increased clearances. Maximum clearance is normally about,006in. and when this is exceeded both rotors must be changed as a pair. Wear can also occur on the peaks in the pump body, in which case a new pump is required.
4. The Rotary-Plunger Oil Pump
A less common type of oil pump is the rotary-plunger type of pump. This has only one moving part: a worm gear rotated a rotary plunger. A peg engaged a profiled groove in the rotary plunger to provide the reciprocating movement, so producing the pumping action. A non-return valve was also necessary. The oil flow from this type of pump was intermittent and the pressure produced was generally lower than that produced by the gear or rotor-type oil pumps. It was therefore inherently unsuitable for many engines, but was used in some engines which utilized roller or ball-races as big-end or main bearings, and as such required only minimal lubrication.
In rare instances, a plunger oil pump was reciprocated by an eccentric peg on the end of a shaft, which engaged a slot in the plunger. A particular disadvantage of both the gear and rotor-type oil pumps was that after the engine had been stopped, oil could drain back through the system to the sump. Neither of these was therefore entirely suitable for a dry-sump lubrication system which might as an alternative employ a form of plunger pump.
5. The Sliding-Vane Oil Pump
The sliding vane pump consists of a rotor set eccentrically in a bore machined in the pump body. This rotor carries a number of sliding vanes, normally four. Each vane is located in a groove and, because of the eccentric setting of the rotor, contact forces the vanes to retract. The outer edge of the vane maintains contact with the pump body and therefore the pump is always divided into three chambers of continuously changing volume. As the vanes pass the inlet port oil is sucked into space and as the size of the chamber decreases the trapped oil becomes pressurized. Therefore, when the vanes pass the outlet port oil is forced into the engine under pressure. This type of pump is highly efficient and has a low leakage rate, but the comparatively large number of moving parts makes it susceptible to wear.
Most modem internal-combustion engines use a wet-sump lubrication system and the engine bearings are usually of the plain white-metal type. These bearings require only a comparatively low-pressure oil supply, of the order of a few pounds per square inch, to provide adequate lubrication. Oil supplied to a bearing at one point is drawn around between the two bearing surfaces by rotation. In this way, a wedge of oil is formed which is of sufficient strength to take the full load of the bearings. The high oil pressure produced by a gear, rotor, or vane pump is used to provide a sufficiently fast flow of oil to absorb and carry away the heat from the bearing surfaces and other parts of the engine such as the pistons, which would otherwise overheat.
Those engines which use a dry-sump system normally have dual oil pumps. One of these is used to provide oil pressure for lubrication. The second (the ‘scavenge’ pump) is used to remove the oil from the sump to the oil tank. To prevent a surplus of oil accumulating in the sump, the second pump usually has a larger capacity than the lubricating pump. Oil pumps require no routine maintenance, although if they incorporate a strainer this should be cleaned periodically. The job needs only be done infrequently, when the sump is removed, and only when removing sludge from the sump.
Oil pumps are usually driven from the camshaft or crankshaft through suitable gearing. It is this gearing which is likely to be damaged if the oil-pump outlet has been blocked and the pump has suffered from over-pressure. It might be thought that constantly being immersed in oil, oil pumps would have an almost indefinite working life. In fact, they have a long working life, but they do eventually wear. As a result, the oil pump output will in time become insufficient for ‘the requirements of the engine. Most service manuals specify maximum limits for wear between the components of the oil pump. If these wear limits are exceeded, it is normal practice to change the oil pump as an assembly. Such renewal is generally only undertaken as part of a major engine overhaul or reconditioning.
6. The Double Gearwheel Oil Pump
The double gearwheel type of pump is infrequently employed in modern cars, although the Audi 100 for example still uses it. The principle of this type of pump is similar to that of eccentric rotor type, the difference being that oil is pumped by spaces formed between gear-teeth instead of by lobed rotors. The two gearwheels have meshed together. One is driven from the camshaft and the other, the idler wheel, rotates in turn. These gears are housed in the pump body with only a small clearance between the body and gearwheels. Inlet and delivery ports are provided at opposite sides of the pumps. The gearwheels rotate, forming a depression over the inlet port and drawing oil up into the pump.
Oil is then carried around the pump in the annular space between adjacent gear teeth. The meshing of the gearwheels at the other side of the pump forces oil out of the tooth spaces into the delivery port and into the lubrication system. In some gearwheel pumps, the driven gearwheel has one tooth fewer than the idler, to ensure even wear of the gearwheels. This type of pump is simple and reliable, except for a tendency to leak at high pressures, although this can be overcome by mounting the pump low in the crankcase where it is immersed in oil. But it is 25 percent less efficient than the rotor pump and has therefore largely been superseded.
7. Dry Sump Oil Pumps
To prevent oil surge during fast driving many races and rally cars employ dry-sump lubrication. In a dry-sump system, oil is stored in a separate oil tank rather than in the sump and therefore two oil pumps are required. One, the pressure pump, forces oil round the lubrication system, while the other, the scavenge pump, returns the oil to the oil tank from the sump. Dry sump oil pumps are invariable of the eccentric rotor type and normal practice is to combine the two pumps in a single unit so that they can be driven by the same shaft.
The inlet port of the scavenge pump draws oil from the sump and forces it through the oil cooler into the oil tank. Oil is drawn out of the tank by the pressure pump and forced into the lubrication system. Some dry pump systems require two scavenge pumps to return all the oil from the sump. In this case, the third pair of rotors is added to the oil pump and the inlet pipe draws from the other end of the sump. The eccentric rotor oil pump is compact and simple and it is possible to drive several sets of rotors off the same shaft. In some specialist oil pumps for racing cars, the pressure pump has five sets of rotors. This type of multistage pump uses successive sets of rotors to produce extremely high pressures.